1. Field of the Invention
The present invention relates to a flat sealed type battery with an electrolyte charging aperture formed in the central portion of a bottom wall of a battery vessel.
2. Description of the Prior Art
In recent years, in various fields there have been used lithium batteries having little self-discharge and a long life with the progress of electronic apparatuses. Therefore, there has been developed a lithium-oxyhalide battery. such as a lithium thionyl chloride battery, adopting a so called hermetic sealing structure of a cylindrical type with a battery lid made of metal-glass-metal, which is used as a power source for memory back up of C-MOS RAM. Since such a battery has a structure with a high sealing function and has a long life of more than ten years, the demand for such a lithium oxyhalide battery is rapidly increased.
However, in the market, not only are the above mentioned cylindrical type batteries for memory back-up in demand but also small, and thin type batteries for memory back-up are also required because of demand for reduction of consumption of current of IC and for miniaturization in size and reducing the weight of the apparatuses.
Although there has been commercialized a flat type lithium battery, such as a lithium manganese dioxide battery or lithium graphite fluoride battery in the prior art, the sealing structure of such a battery is made in so called a crimp seal method that there is provided a gasket made of a plastic resin material between an opening end portion of a positive electrode can and the outer peripheral portion of a negative electrode can so that the opening end portion of the positive electrode can is fastened inwardly so as to seal the battery vessel (see the Japanese Patent Laid Open Sho 56-167274 for example). Therefore, there is a limitation period in which the battery is available with a sealing ability, and the battery is available for, at most, 5 to 7 years and it can not possibly be used for more than ten years. Accordingly, though a flat type sealed battery with a high sealing function, employing a hermetic seal, is in demanded by users, the demand of the users has not yet been realized due to the difficulty of the sealing techniques after charging the electrolyte.
That is to say, in the cylindrical type battery, the size and volume of the battery is large and the total height of the battery is at least more than 25 mm and since the sealing process is performed on the top end portion of a pipe used as an electrolyte charging entrance after the charge of the electrolyte (see the Japanese Patent Laid Open Sho 62-160660 for example), therefore, there can be formed a distance of at least more than 5 mm between the surface of the electrolyte and the sealed portion in the battery vessel, so that the heat applied at the time of the welding for sealing has a little affect on the electrolyte. However, in the flat type battery with a total height of the battery of at most 10 mm, since the distance between the surface of the electrolyte and the welded portion for sealing is merely about 1 to 2 mm, the electrolyte is vaporized due to the heat applied at the time of welding for sealing so that the vaporized gas is spurted out from the inside of the battery, which flies to the welded portion for sealing, so that there occurs a pin hole at the welded portion, resulting in that a perfectly sealed structure can not be realized.
In the conventional flat type battery, if an electrolyte charging entrance is formed at the edge portion of the battery lid, the welding portion thereof is so near to a glass layer that the glass layer can easily be broken due to the heat of the welding, and therefore, as shown in FIG. 1, it has been attempted to form a hole at the central portion of the bottom wall 5a of the battery vessel 5 to use as the electrolyte charging entrance 12. After the electrolyte is charged into the battery vessel through the electrolyte charging entrance 12, the state of the battery is reversed so as to be turned upside down to the state of the battery shown in FIG. 1 at least from the time of charging the electrolyte and covering the electrolyte charging entrance 12 with a sealing plate 14 and the outer peripheral portion of the sealing plate 14 is welded with the bottom wall 5a of the battery vessel 5. However, as described above, since the distance between the welded portion for sealing and the surface of the electrolyte is so short the electrolyte is vaporized due to the heat of the welding and the vaporized gas escapes out of the welded portion, resulting in that the welding is prevented or there occurs a pin hole at the welded portion so as to cause damage to the sealing structure.